Steady-state and pre-steady-state kinetic studies on the interaction of actin, myosin-subfragment-one (S-1) and ATP were carried out to better elucidate the mechanism of the actin activated myosin ATPase. In particular, we tested whether there is mandatory detachment of myosin from actin during each cycle of ATP hydrolysis as proposed by the Lymn-Taylor model. Our studies on the rate and magnitude of the ATP hydrolysis step (initial Pi burst) in the presence of actin are in strong contradiction to the predictions of the Lymn-Taylor model that this step only occurs when the myosin-ATP is detached from actin. We find that the Lymn-Taylor model cannot simultaneously fit the steady state and pre-steady-state kinetic data. Therefore our results show that myosin does not detach from actin during each cylce of ATP hydrolysis; the ATP hydrolysis step occurs at an even faster rate when the myosin ATP is bound to actin than when it is dissociated from actin.